The nature of the lithosphere beneath the Sardinian continental block: Mantle and deep crustal inclusions in mafic alkaline lavas

Abstract

Ultramafic and mafic rocks are abundant as inclusions in Plio-Pleistocene alkali basalt, trachybasalt and basanite magmas. They provide an important “window” into the nature of the lithosphere beneath the Sardinian continental block. The inclusions are predominantly porphyroclastic-textured spinel peridotites, although pyroxenites, mafic granulite-facies gneisses and augite megacrysts also occur. Equilibration temperatures of 956 ± 34°C, using the Wells geothermometer, imply that the peridotites were entrained by rising alkalic magmas somewhere near the top of the mantle lithosphere. Pyroxenites and augite megacrysts are more Fe-rich and have lower CrAl ratios than clinopyroxenes from peridotite, and may have precipitated from mafic magmas at mantle pressures. The available evidence strongly implies considerable chemical and mineralogical heterogeneity in the upper mantle lithosphere beneath Sardinia. Diopside mineral separates from two spinel peridotite xenoliths are characterised by (LaYb)_N > 1 and a chondrite-normalised U-shaped REE pattern, which cannot be explained by a single stage partial melting event. Trace element constraints imply that the rare samples of the deep crust, now granulite-facies gneisses, were originally gabbroic cumulates.